success

There is only one measure of success for a composer.  And it is impossible to prove.

Success is:  performers you don't know deciding to play your piece.

Look carefully, and you will see that most performances in the world today, including the very visible, very high profile ones, are the result of direct personal connections.  At the highest levels, they are often compounded by business connections (friends being managed by the same company, for example.)  Or they occur because one set of managers talks to another set of managers.

Is that success?  Of a sort, certainly.

But real musical success happens when you send a work on its solitary way into the world and somewhere, performers who don't know you personally decide to play it.

Why can't you prove it?

Because you simply never know what wheels there are within wheels.  You may see a list of performers who just did your work and not know any of them, but that doesn't mean they don't know someone you know.  You can never know if one of your friends was responsible for the connection.  Perhaps one of the performers was a student of someone you know.

Of course, that doesn't take away from the fact that people who don't know you decided to perform the work anyway.

The extension of this definition of success is being asked to compose a new work by someone you don't know.  Someone has either performed your existing work or knows it somehow, and wants a new piece.

It happens.